Synthetic as well as natural polymeric materials are degraded in appearance and properties by exposure to high-energy solar radiation, especially that occurring in the ultraviolet or near ultraviolet region of the spectrum. The energy content of the light in this region is sufficient to cause breakage of bonds in organic molecules, including polymeric molecules.
Absorption of light by a polymeric molecule ultimately may cause the polymer to become crazed, to chalk, to become discolored, to lose gloss, to have a reduction of physical properties, or a combination of two or more of these effects. Continued exposure eventually may lead to embrittlement. However, polymers react to light in different ways. Polymers such as polyvinyl fluoride are quite resistant to light deterioration, whereas polyolefins deteriorate rapidly.
Through the years many different types of stabilizers for polymers have been utilized with varying degrees of success. Benzophenones, benzotriazoles, and aryl esters of organic acids are some of the earlier ultraviolet stabilizers used. These compounds differed as to the type of polymeric compound they protected and against which wavelengths they were effective.
The prior art compounds, such as the benzophenones and benzotriazoles, act as light stabilizers through the preferential absorption of light in the ultraviolet or near ultraviolet region of the spectrum. The energy of the light is dissipated as heat by the stabilizer, without damage to the polymer. However, these U.V. absorbers are useful only in articles with a substantial cross section, to allow for the light absorption at reasonable use concentrations, and are not useful for fine fibers or thin films of the polymer. The aryl esters of organic acids are also U.V. absorbers, and are slowly destroyed on exposure to light, thus their effectiveness diminishes over a period of time.
Piperidine and piperazine derivatives have also been utilized as ultraviolet stabilizers with some degree of success, as in U.S. Pat. Nos. 3,899,491, 3,928,330, and 4,007,156, but still retain some of the disadvantages of their predecessors.
Hexahydropyrimidines have been utilized, in the past, as antioxidants in natural and synthetic rubbers, see U.S. Pat. No. 4,104,249. Hexahydropyrimidines have also been utilized as accelerators for rubber vulcanization, a cross-linking reaction, which would appear to suggest a property which is opposite to the light stabilization property of this invention.
In many cases natural and synthetic rubbers, such as those with which U.S. Pat. No. 4,104,249 is concerned, include carbon black as one of the formulation components. The carbon black present has many functions including being an ultraviolet light absorber. There was no necessity therefore for the anti-oxidant to be also an ultraviolet absorber or for the composition to include a separate, added light absorber. Compounds normally do not possess both anti-oxidant and ultraviolet light absorption properties.
The hexahydropyrimidines of U.S. Pat. No. 4,104,249 are very susceptible to hydrolysis in the air during storage and compounding and also to degradation during processing. The compounds were not considered useful as light stabilizers. In addition to their instability due to hydrolysis, the compounds were of low molecular weight, leading to undesirable volatility during processing.
Hexahydropyrimidines have also been utilized as bactericides and fungicides, U.S. Pat. Nos. 3,872,120 and 3,875,159; as corrosion inhibitors, U.S. Pat. No. 4,146,714; and for their pharmacological activity.
U.S. Pat. No. 3,910,948 describes N,N'-diacyl derivatives of hexahydropyrimidine, and discloses that they are useful as mold growth inhibitors. The compounds disclosed exhibited both selective and broad antimycotic activity, depending upon chemical structure. Some of the compounds exemplified were 1,3-diacetylhexahydropyrimidine, 1,3-dibutyrylhexahydropyrimidine, 1,3-dipalmitoylhexahydropyrimidine, and the like.
Japanese Patent Publication (Kokai) No. 1974-53575, published May 24, 1972, describes anti-deterioration agents that are suggested for use in polyethylene, polypropylene, many vinyl polymers, polyesters, polystyrenes, and polyamides. These agents are apparently intended to improve the light stability of the polymers. The agents are 4,6,6-trimethylhexahydropyrimidine derivatives of the general formula ##STR5## where R.sub.1 and R.sub.2 are the same or different alkyl groups, allyl groups, or aralkyl groups, which groups have from 1 to 18 carbons, and where R.sub.1 and R.sub.2, along with the ring carbon to which they are attached, may together form a 4- to 7-membered alicyclic ring, or with a hetero atom may constitute a hetero-alicyclic structure. Structures such as these are very susceptible to hydrolysis, regenerating R.sub.1 R.sub.2 C.dbd.O and the corresponding diamine. Hence they are subject to rapid degradation during handling. In addition, when R.sub.1 and R.sub.2 are lower alkyl groups, the structure is likely to be lost easily from the host polymer matrix through volatilization, because of the low molecular weight of the structure.
Some of the chemistry and chemical reactions of hexahydropyrimidines are described in Helvetica Chimica Acta 30, 1114 (1947), and J. Prakt. Chemie 38, 339 (1965).